The use of carding machines for forming nonwoven webs of staple-length fibers, oriented in the machine direction of web formation, are well known in the prior art. Such a machine and method of forming a web is disclosed in U.S. Pat. No. 3,772,107--Gentile, et al. As disclosed in Gentile, et al. as well as in U.S. Pat. No. 4,566,154--Streeper, et al. it is common to use a web spreading apparatus to increase the cross machine width of the web coming out of the card line by as much as 45 percent. In the older carding machines, the fibers in the formed web were very highly oriented in the machine direction of web formation. Randomizing rolls have been developed which greatly reduce the tendency of the fibers in the formed web to be oriented in the machine direction of web formation. A problem that has been encountered in using the randomizing rolls to reduce the machine direction orientation of the fibers in the web is that it becomes more difficult to remove the formed web from the forming surface for further processing.
U.S. Pat. No. 4,425,126--Butterworth, et al. discloses a process for making a fibrous web in which 10 percent or more by weight of the fibers are synthetic wood pulp fibers formed of polyethylene. The web is then heated to a temperature above the melting point of the synthetic wood pulp fibers but below the melting point of the other fibers in the web thereby causing the synthetic wood pulp fibers to be fused and bonded with each other and with at least some of the other fibers in the web. The stabilized web of Butterworth, et al. is then subjected to a second bonding step with adhesive (latex). As stated at column 3, lines 42-47, the bonds formed by the fused synthetic wood pulp fibers greatly reduces, if not eliminates the wet collapse of the web when the aqueous latex solution is applied to the web during the adhesive bonding step.
U.S. Pat. No. 3,989,788--Estes, et al. also discloses a process in which a web undergoes a first stabilizing bonding step followed by a second bonding step. During the first bonding or consolidation step, the web is heated so that the binder filaments become tacky generating some binder-to-binder bonds and binder-to-matrix bonds. The matrix filaments are not appreciably affected by the consolidation step. The web is then subjected to a second bonding step at a higher temperature, that will melt the binder fibers so that they lose their filamentary form and act as an adhesive, but which has only a slight softening on the matrix fibers.
Applicant has found that in trying to apply the technology of Butterworth, et al. and Estes, et al. to the web forming process as disclosed in U.S. Pat. No. 4,315,965--Mason, et al. by adding polyethylene binder fibers on the order of 10 percent or more by weight of the finished web to polypropylene fibers, the strength of the thermally bonded web is decreased significantly.
Furthermore, it is not obvious that one could stabilize the web by using bonding fibers and still be able to spread the stabilized web.
It is, therefore, one object of this invention to provide a method of stabilizing an unbonded web by using secondary bonding fibers and then thermally bonding the stabilized web, in which all of the strength of the thermally bonded web is due to the fiber-to-fiber bonding of the primary fibers.
It is another object of this invention to stabilize an unbonded web in a manner that does not otherwise materially reduce the strength of the thermally bonded web.
And yet another object of this invention is to provide a method of stabilizing an unbonded web by using secondary bonding fibers and then spreading the stabilized web.